When welding seams in a resistance-roller welding, there are generated temperatures in the clamping region of the welding location between two electrode rollers in excess of 1,000 degree C., so as to ensure a trouble free connection between two portions of sheet metal. This high temperature increases the velocity of oxidation of the surface of the weld seam to such an extent, that a thick oxidation layer results, which adheres only poorly to the sheet metal, is very brittle, and can easily become detached during further processing of the can components, for example when stiffening the can obtained from the can components, or providing it with a rim. A further disadvantage is the fact that any layer of lacquer subsequently supplied to the welding seam may be broken, and therefore loses its function as protection from corrosion.
These problems have been recognized, and it has already been proposed to seal the region of welding, namely the clamping region between the welding rollers, with respect to the surrounding atmosphere as well as possible, and to supply an inert gas into the sealed space (compare German laid-open patent application DE-OS No. 2908574).
In another known arrangement, namely U.S. Pat. No. 3,652,818, it is proposed to provide in a wide region upstream from the electrode rollers a space sealed from the surrounding atmosphere for containing an inert gas, so that no air or oxygen arriving at the welding location can reach any overlapping portions of the sheet metal.
These two known arrangements achieve their objectives, but require relatively expensive sealing means to maintain the inert atmosphere in the above-described regions, so as to avoid an unnecessarily large and uneconomic amount of the expensive inert protecting gas.
The known arrangements have a further disadvantage that they cannot be retrofitted easily to existing machines. Nor are they suitable for machines where the diameter of the can is changed frequently.